1. Field of the Invention
The present invention pertains to coating processes using vapor deposition of coatings including diamond.
2. Description of the Prior Art
The carbon allotrope diamond has many potential uses. Diamond has extreme hardness, resistance to thermal and mechanical shock, and transparency to a wide range of electromagnetic wavelengths from ultraviolet through visible and infrared radiation to microwaves. Diamond would thus be an unexcelled coating for transmitting, reflecting, and absorbing optical and microwave elements subjected to weather, particulate erosion, and high energy radiation. Diamond also has high thermal conductivity, high hole mobility, and high electrical resistivity when pure, and functions as a semiconductor when doped. Diamond would thus make integrated circuits and other electrical and electronic devices incorporating a diamond film unexcelled for operation at high temperature, for convenience of cooling, and where transparency, environmental resistance, and radiation resistance are desirable.
The chemical vapor deposition (CVD) of a film of polycrystalline diamond from an activated gaseous mixture which includes a gas containing carbon is well-known and would appear to make these potential uses practical However, diamond films deposited heretofore by CVD on a non-diamond substrate do not adhere thereto sufficiently for practical purposes unless the substrate, is first seeded with diamond particles as by abrasion of the substrate with diamond grit. As a result of the abrasion and seed particles, the substrate is irregular and the crystallites forming the deposited film are irregular in size and spacing, very defected, and without preferred crystal orientation. The substrate and diamond film of the prior art are thus too irregular for use as an optical coating although continuous and smooth polycrystalline films are well-suited as optical coatings. Although prior art CVD diamond may be usable as a relatively massive heat sink, prior art CVD diamond is too irregular for use as an electrical insulating or thermal conducting layer within an integrated circuit.
The irregularities of prior art CVD diamond films may be disadvantageous for mechanical protection even where optical and electrical properties are irrelevant. For example, a very thick diamond layer may be rapidly deposited using a plasma torch or jet in which the carbon containing gas, which may be a portion of a flame, is activated by discharging the gas through an electric arc. However, the resulting diamond layer is so irregular and the crystallites so imperfectly joined that the layer has, despite its thickness, relatively poor resistance to weathering.
The deposition, in a chamber containing gas at a pressure less than atmospheric, of a film or layer of a material onto a substrate is, of course, well-known. Such vacuum deposition may be carried out by sputtering where ions of a gas, typically argon heated by microwave energy, eject atoms to be deposited from a target of the material so that the freed atoms travel to an adjacent substrate and are deposited thereon. Movement of such freed atoms to the substrate may be motivated by a suitable electric potential between the target and substrate. An oxide or nitride of the target material may be deposited by including, respectively, oxygen or nitrogen in the gas in the chamber. Suitable materials, structures, temperatures, and pressures for sputtering deposition of oxides and nitrides of a variety of elements on a variety of substrate materials are readily available for selection by one skilled in the art of vacuum deposition.
In chemical vapor deposition, atoms to be deposited on a substrate are provided as atoms in molecules of a gas present in the chamber and activated while in contact with the substrate. Typically, the gas is activated by heating the gas by microwave energy, a hot filament, electric discharge, or combustion so that the gas releases free radicals containing the atoms to be deposited on the substrate. Typically in CVD, no electric potential relative to the substrate is provided, and the substrate is maintained at a suitable temperature by electrical resistance or induction heating.
In deposition of diamond by CVD, a gas containing the carbon which forms the diamond is provided as small proportion of a gaseous mixture in the chamber, the balance of the gas being predominately hydrogen Such a mixture may be activated by microwave energy at a frequency which excites the hydrogen molecule. The carbon containing gas is usually methane which is readily obtained in a pure state and is present in the mixture at a proportion less than 5% and, typically, 0.5% to 2%. However, the necessary carbon-containing free radicals may be obtained from vapors of other hydrocarbons, alcohols, or the like. It is known in diamond CVD to add a small amount of oxygen to the mixture of hydrogen and carbon containing gas, the proportion of oxygen being substantially less than that of methane. The oxygen serves to increase the rate and quality of diamond deposition by oxidizing graphite which, depending on the deposition conditions, may be deposited along with the diamond.
Diamond may be deposited by CVD over a wide range of conditions. The vacuum chamber may be maintained at a pressure of 0.1 to 100 Torr by pumping while providing new gaseous mixture. The substrate is maintained at 550.degree. to 1100.degree. C. In general higher pressures increase the rate of diamond deposition as do higher substrate temperatures up to 900.degree. or 1000.degree. C. In prior art diamond CVD, variations in substrate temperature, in gas activation method and temperature, in the proportion of carbon providing gas, and in the method of substrate abrasion provide some control over the crystallite size and growth rate size and over the irregularity of the deposited diamond. However, in all prior art diamond CVD the deposited diamond is, as before stated, highly irregular.
It is apparent that, in optical applications of a polycrystalline film and even if crystallites thereof are regular and substantially undefected, smaller crystallites are desirable to reduce scattering. However, CVD diamond film deposition conditions that provide smaller crystallites also generally result in slower film growth rate. It would, therefore, be highly desirable, in a method for CVD deposition of optical quality diamond film, to provide for deposition of very small crystallites. It would be particularly desirable in such a method to grow a film of small crystallites without prohibitive reduction in film growth rate and to provide, selectively, for more rapid growth of a film of larger but still regular crystallites where these are acceptable.